Piston pump



FIG. 1 is a front view of a piston pump showing our new design;

FIG. 2 is a rear view thereof;

FIG. 3 is a bottom plan view;

FIG. 4 is a top plan view thereof;

FIG. 5 is a right side view thereof;

FIG. 6 is a left side view thereof; and,

FIG. 7 is a perspective view thereof. 

The ornamental design for a “piston pump,” as shown and described. 